When it was first talked about on x299, it was said to use a fourier transform algorithm similar to what prime95 uses. No mention of frequency resolution or instruction sets used though, I doubt its AVX-512. I don't think whoever did the stream I saw really knew the specifics of what it did to stress the CPU but theoretically it could put a pure load on the CPU such that you can insure stability a lot faster than in windows because of how clock scheduling works.
 
I really like the idea of in-BIOS stress testing.. Really wish it'd come to z370, but it's unlikely since its seen as a premier feature for z390.